1. Field of the Invention
The present invention relates to an apparatus for continuously laying a pipeline on the floor of a body of water from a pipelaying vessel. The apparatus comprises a reel upon which a pipeline to be laid is wound. The apparatus also includes a stinger or other curved path pipeline guiding arrangement (e.g. a curved track, a wheel or chute) provided at the stern of the vessel for guiding the lowered pipeline. The apparatus further includes launch means for the controlled lowering of the pipeline over the stinger of said vessel and into the body of water.
2. Description of Background Art
Such a vessel is well known, e.g. from U.S. Pat. No. 3,965,713. A reel with a substantially vertical axis is provided, and the launch means in this embodiment are formed by tensioning means. During pipelaying, it is important to control the pipeline tension in a predetermined tension range. There is a risk of overstressing (too much tension in a pipe) or buckling (too low tension) in the pipeline section that has left the reel and has not yet reached the floor of the body of water. In particular the pipeline sections in the vicinity of the floor of the body of water and in the vicinity of the surface of the body of water can be subjected to stresses deviating from what is acceptable.
To keep the vessel at a desired position with respect to the lowered pipeline, often anchors are used. Multiple anchors and anchor handler vessel(s) are then required to perform the pipelaying operation and to control the position of the vessel accurately. A large offset is not desired for pipelaying operations, because it will result in pipeline tension outside the predetermined allowable tension range. Hence, accurate anchoring is required to control the pipeline tension.
A disadvantage of such a positioning system is that anchor handling is time-consuming and complex.
Therefor, alternatively the pipelaying vessel is kept in place by a dynamic positioning system controlling a suitable propulsion system of the vessel, e.g. with multiple thrusters, such as described in U.S. Pat. No. 6,328,502. However, a disadvantage of such systems is the limited accuracy. The actual position of the vessel may vary up to several meters offset from the desired position, which is found to be problematic in pipelaying, in particular when pipelaying is relatively shallow water, e.g. the North Sea.